Definitions of some of the technical terms used in the rest of the specification are given below.
A polymer is a class of materials having structures that contain repeating structure units covalently bonded to each other. The repeatable structure units, which are called monomers, can be identical or different from each other. The reaction from the monomers to form the corresponding polymer is called polymerization.
A hardener/curing agent is a chemical substance or mixture added to a resin to harden it by forming a polymer. Generally, it will promote or control the hardening or curing reaction of the resin. An agent which does not enter into the reaction is known as a catalytic hardener or catalyst. A reactive curing agent or hardener is generally used in much greater amounts than a catalyst, and actually enters into the reaction.
“Self-healing” is the ability to repair damage and restore lost or degraded properties or performance of a material using resources inherently available to the system. This concept is inspired by biological systems which can heal themselves after being wounded. Currently this function is mainly realized in polymer-based materials by either the incorporation of heterogeneous healing carriers or by molecular design. Materials which have a self-healing functionality are called self-healing materials. The first generation of self-healing material was patented in the United States of America and given U.S. Pat. No. 6,518,330 B2.
“Anticorrosion” is the ability to prevent metal or alloy surfaces from corrosion or to retard the corrosion process of the metal surfaces in a corrosive environment. Anticorrosion can be achieved through surface treatment physically or chemically. The anticorrosion species are one class of chemicals which are adopted to realize the retardation of corrosion by forming new materials on the metal surface first before corrosion happens.
Hollow glass beads (HGBs) are a newly developed material made of borosilicate glass with a thin shell and a relatively high cavity volume. Because of their excellent properties, such as low density, low thermal conductivity, high strength and good chemical stability, they have been extensively applied in areas ranging from aerospace to high-speed train to sports equipment. After modification of their hydrophilism, they can be easily and readily incorporated into polymeric matrices to achieve composites with special properties.
Because of their relatively high volume cavities inside the shell, HGBs can be adopted as gas carriers for some simple and small molecules. In 1994, Akunets et al [1] reported the storage of hydrogen using hollow glass balloons with relatively thicker shells because the pressure inside is very huge. Since the hydrogen molecule is very small and it is the simplest molecule, they can diffuse into the cavity through the dense wall under special circumstance. HGBs can be also filled with neon or deuterium to be applied in the laser fusion area. However, when intact HGBs are used without modification, the filling process involving high temperature up to 300° C. and high temperature up to 100 MPa is very challenging and not feasible for large scale applications.
Given the good properties of HGBs, such as high strength, good chemical inertness and thermal stability, they could also be adopted as potential microcontainers for various substances, including reactive chemicals, anticorrosive species, catalyst, drug, etc. In this field, it is required to have ready and easy loading of the desired chemicals in their preferred states with controllable release rate of the loaded chemicals from the container. As mentioned above, without modification of the shell structure, the filling of HGBs with liquid or solid is much more difficult even under high temperature and high pressure for long time. The direct fabrication of the porous shell HGBs is an alternative method to produce this kind of containers (US20100139320, U.S. Pat. No. 4,637,990, and U.S. Pat. No. 4,793,980). However, because the porous shells fabricated in these prior art methods have only nano-channels or subnano-channels, the filling and the release of this kind of porous HGBs are still issues of concern.
After proper modification of their shell structure, HGBs might be used as microcontainers for highly reactive agents for self-healing materials, including composites and coatings. Self-healing is the ability to recover the functionality for the materials without human intervention, which is being deeply investigated since the last decade. Among the developed mechanisms for self-healing, the microencapsulation of reactive healants is a major approach. Mature methods employing microcapsules to fulfill the self-healing functionality include the use of dicyclopentadiene (DCPD)/Grubb's catalyst, polysiloxane/tin catalyst, diisocyanate, epoxy/hardener, etc. As epoxy resin is widely used as matrix material for composites for protection and for adhering the reinforcements because of its excellent physical and chemical properties, self-healing of the brittle epoxy matrix is attracting more and more attentions. Use of epoxy-amine two part healing chemistry is preferable in epoxy based composites, which can keep excellent materials compatibility and low cost. However, the fabrication of healing containers with primary amines is very difficult due to their reactive feature and ease of solubility in most solvents. In existing epoxy-hardener systems, some secondary classes of hardeners, like polythiol, latent catalysts, cationic catalysts, were explored rather than the major class, primary amines and their derivatives. The direct microencapsulation of diethylenetriamine (DETA) was indeed reported by Mcllroy et al [2]. However, their usage to realize the self-healing functionality has not yet materialized. Recently, Jin et al. [3] reported microcapsules containing an amine derivative using a two-step method by first synthesizing hollow polymeric microcapsules and then loading them with the amine via vacuum infiltration. However, the long-term stability of the polymeric shell was diminished by the corrosive amine and the thermal stability of the loaded amine under elevated temperature still needs further improvement. How to fabricate the healing containers for highly reactive and corrosive amines with reasonable stability is a breakthrough that will advance the more practical healing chemistry in the field.
Self-healing coatings by the incorporation of microcapsules containing healants, such as diisocyanates [4, 5] and polysilane [6], have been explored recently. The mechanism for these functional coatings lies in that the encapsulated healants can react with water to form a solid material in the damaged area to impede or even prevent the corrosion of the coated substrates. As isocyanates can react with reagents with active hydrogen atoms, such as polyols to form polyurethane, polyamine to form polyurea, or just water and moisture, it is a challenge to encapsulate the diisocyanates. Up to now, the successful microencapsulation of diisocyanates was only reported by the Yang group when they used interfacial polymerization of isocyanate prepolymers with diol to encapsulate isophorone diisocyanate (IPDI) and hexamethylene diisocyanate (HDI), as filed in U.S. Patent application No. 61/593,530. However, the high permeability of the loose polymeric shell of the microcapsules limits their application because the solvent or any compounds with active hydrogen atoms in the surroundings would deactivate the reactive diisocyanate.
The modification of HGBs can be an alternative way to fabricate the microcontainers for anticorrosive species, drugs, as well as healants, given the properties of the glass shell. Two potential etching methods can be used to achieve the modified HGBs: concentrated alkaline solution such as potassium hydroxide under elevated temperature or diluted hydrofluoric acid (HF) solution at room temperature. Because the etching process using the alkaline solution is very slow [7], the second method using diluted HF solution seems more attractive. To improve the performance of the lead-acid battery by rapidly transportation of the electrolyte, Newell et al. [8] etched the HGBs as the vehicles for the electrolyte by directly putting HGBs into diluted HF solution and shaking the mixture for a certain time. However, this is not a controllable process for the HGBs. It acts in an undesirable manner as through-shell etching of the HGBs will accelerate the etching of already well-etched beads because the etching reaction would take place both outside and inside the HGBs, leading to over-etching.